Thyroxine and reverse T3 5'deiodination in the cells of human adipose tissue.

The intracellular 5'deiodination (5'D) of T4 and rT3 has been investigated in human adipose tissue. We studied 5'D in intact adipose tissue, its morphological components and in 3T3-L1-cells. 5'D as assessed by T3-production out of T4 and by rT3-decomposition was not inhibited by propylthiouracil (PTU), but by iopodate (IOP). The apparent Michaelis constants were kM = 3 nM for rT3 and kM = 1 microM for T4. The rT3-degradation was linear over 25 h (115 pg/h.mg (prot.)) both at 37 degrees C and at 4 degrees C. The same type of 5'D was observed in adipocytes, stromal-vascular cells and in 3T3-L1-cells regarding T4 to T3 degradation (244 +/- 30, 181 +/- 27, 227 +/- 37 pg T3/mg.min), resp.; PTU did not exert any influence upon 5'D in the cells investigated. We conclude, that i. the intracellular generation of T3 in adipose tissue does not derive from type I deiodination; ii. 5'D in adipocyte precursors and differentiated adipocytes is identical and iii. there is no difference between human cells and 3T3-L1-cells regarding 5'D.